Why didn't the angel in Acts 5 simply stop the arrest of the Apostles? Why did he wait for them to be arrested and then release them in the middle of the night? At least 2 reasons:

1) Another miracle to build the faithful and harden the unfaithful
2) An object lesson of death and resurrection

The angel could have leveled the city, freed every prisoner, and killed every enemy of justice, but that wouldn't have served God's purposes at the time. Instead, he only temporarily and selectively blinded the guards and unlocked the gate. He didn't destroy any property or harm anyone, and he left the other prisoners behind.